Ocean mass, together with steric sea level, are the key components of total observed sea level change. Monthly observations from the Gravity Recovery and Climate Experiment (GRACE) can provide estimates of the ocean mass component of the sea level budget, but full use of the data requires a detailed understanding of its errors and biases. We have examined trends in ocean mass calculated from 6 yr of GRACE data and found differences of up to 1 mm yr−1 between estimates derived from different GRACE processing centre solutions. In addition, variations in post-processing masking and filtering procedures required to convert the GRACE data into ocean mass lead to trend differences of up to 0.5 mm yr−1. Necessary external model adjustments add to these uncertainties, with reported postglacial rebound corrections differing by as much as 1 mm yr−1. Disagreement in the regional trends between the GRACE processing centres is most noticeably in areas south of Greenland, and in the southeast and northwest Pacific Ocean. Non-ocean signals, such as in the Indian Ocean due to the 2004 Sumatran–Andean earthquake, and near Greenland and West Antarctica due to land signal leakage, can also corrupt the ocean trend estimates. Based on our analyses, formal errors may not capture the true uncertainty in either regional or global ocean mass trends derived from GRACE.